(1) Field of the Invention
The present invention relates to a technology for transmitting/receiving digital data of a work such as a movie or music (hereinafter, such digital data is referred to as “content”), and specifically relates to a technology for preventing a copyright infringement such as copying a content in an unauthorized manner, and, if an unauthorized copy of a content is found to be distributed, identifying a device that generated the unauthorized copy.
(2) Description of the Related Art
In recent years, as the personal use of contents has become prevalent, what are called “pirated contents”, which are generated by copying contents in an unauthorized manner, have come to circulate.
U.S. Pat. No. 6,285,774 discloses a technology for identifying, from an unauthorized copy, a client that has made the unauthorized copy, within a model where a server distributes contents to clients.
More specifically, the server copies a scene within a content and thereby generates n copies of the scene, and embeds different electronic watermarks respectively into the copies of the scene. The server then encrypts each of the plurality of copies using a different encryption key, and distributes the content to the clients. Each client holds one of a plurality of different keys, and plays back the content by decrypting a specified portion of the content using the key.
And then, if a pirated version of the content is found to be distributed, it is possible to identify the client that made the unauthorized copy, by checking the electronic watermark that is embedded in the pirated content.
However, there is a possibility that, although it should not happen, the server performs unauthorized copying, and there is a demand from content holders to prevent the server from performing the unauthorized copying.
Also, recently, in some homes, a plurality of terminal devices held by users are connected to each other via a network, and contents are transferred between the terminal devices. In such a case, each terminal device can be either a server (transmission side) or a client (reception side). This requires a mechanism for preventing the transmission side, as well as the reception side, from performing an unauthorized act.
Here, a problem of the technology disclosed in U.S. Pat. No. 6,285,774 is that it cannot prevent the transmission side from performing an unauthorized act, although it does the reception side.